Is every property of an Entity in domain driven design a value object?

Question

I'm reading "Patterns, Principles, and Practices of Domain-Driven Design". The book suggests that properties of an Entity should be value objects in order to model domain's ubiquities language. I've seen many examples like EmailAddress or Age with only one field to model domain concepts. I'm confused about it. Is every property of an Entity a value object? Can you provide examples when we can use normal languages provided data types for properties?

Answer 1

No, not every property of an entity is a value object.

Properties of entities are one of the following:

• As you already know, value objects. Value objects represent simple values without identity.
• Primitives. These are just value objects from a DDD perspective, really. Primitives are ok to use in DDD, but take care not to become a victim of Primitive Obsession.
• Entities. An entity can contain other entities. All entities that have direct (navigable) references between them are part of the same Aggregate. The "top-most" entity within an aggregate is called the Aggregate Root. Only the root has a global identity, inner entities have only local identity.
• References to entities of other aggregates. Never reference these directly, use an ID. IDs themselves can in turn be modeled as value objects.

Answer 2

I think that your real question is: Is every value object a Class? Because you can think that for the Age a Java Integer can be enough and this is true. So you have in your entity Person a value object Age of type Integer, there is no need of an age type.

OOP also says that an object is state + behaviour. In your Age case, I assume that it has no behavior so a simple primitive or wrapper class will do the trick, in fact I would go with option this because is simpler.

My advise is, go with a primitive/wrapper class and if you advert that some behavior is needed in that value object, make a class/type.